Does anyone know a simple way to adjust effects?

I adjusted the speed of a certain part of a track and now think I overdid it. Is there an easier way to modify the speed of that specific part of the track? Or do I have to delete it and re-add the speed change?

How did you do that?
If you used the “Change Speed” effect to speed up the track, then you can use the same effect to slow down the track. If you wish to affect only part of the track, “select” the part of the track that you wish to process.
See here for how to “select” part of an audio track: